1. Field of the Invention
This invention relates to an electronic picture taking apparatus, and more particularly to a video camera, a digital camera or the like, small in depth in spite of using a zoom lens system as a taking lens.
2. Description of the related Art
In recent years, digital cameras (electronic cameras) have come to attention as a next-generation camera instead of the 35 mm film cameras (commonly called Leica size film camera). Digital cameras are categorized in several classes from a large professional-grade model having extremely high performance to a portable small consumer model.
As to the consumer model, the need for minimization of camera depth has intensified in the marketplace. However, it is difficult to realize such a small camera because the size of the optical system, particularly the distance from the front surface to the image plane of the zoom lens is large. Lately, a so-called collapsible mechanism has come to be mainstream of the camera body structure in which the lens barrel can be telescopically collapsible into the camera body. When the camera is in use, the lens barrel goes into the camera body, reducing its depth. Whereas, when the camera is used for picture taking, the lens barrel comes out of the camera body. However, the depth of the camera body in the collapsed state varies greatly depending on the type of the zoom lens and filters used. When the specification of the zoom lens such as zoom ratio or F-number is set in high level, a positive lead type zoom lens having a positive foremost lens unit tends to have thick lens elements and deep airspace therebetween, and therefore, depth of the camera body does not become small even when the collapsible mechanism is adopted. (Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 11-258507(1999)).
A negative lead type zoom lens having a negative foremost lens unit has no such problem when it consists of only two or three lens units. However, in negative lead type zoom lens systems, the number of lens elements in each lens unit is large or each lens elements is thick, so the depth of camera body does not become very small even if the negative lead type zoom lens and the collapsible mechanism are adopted. (Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 11-52246(1999)).
Conventional zoom lens systems suitable for an electronic picture taking apparatus, having good imaging performance including zoom ratio, angle of view and F-number etc., and being expected to reduce depth of the camera when the lens system is collapsed, are disclosed in Japanese Patent Application Unexamined Publication (KOKAI) Nos. Hei 11-194274(1999), Hei 11-284953(1999) and 2000-9997(2000).
For reducing the thickness of the foremost first lens unit of the zoom lens system, it is desirable to make the position of the entrance pupil shallow, that is, to make the distance from the object side surface of the first lens unit to the entrance pupil small. To realize this, it is necessary to make the magnification of the second lens unit large. However, as the magnification of the second lens unit becomes greater, it becomes difficult to make the thickness of the second lens unit small because of the great share of magnification loaded to the second lens unit, and moreover, the difficulty of manufacture and the sensitivity to manufacturing error increases with undesirable results.
It is possible to minimize the camera depth and size using a small image pickup device. However, to maintain a constant number of pixel element, the pitch and size of pixel elements must be small and the sensitivity of the image pickup device inevitably decreases. Therefore a large aperture zoom lens must be used to compensate for the sensitivity. The adverse effect on resolution due to the diffraction of light also must be compensated for by the zoom lens.
To reduce the depth of camera body, it is effective for the layout of the focusing lens driving mechanism to adopt the rear focusing system in which the lens unit other than the first lens unit is used for focusing. In this case, a zoom lens system having a small change in aberration during focusing is required.
Reduction of the depth of an electronic picture taking apparatus is achieved by reducing the thickness of the optical system. As to the picture taking lens, it is reduced in thickness by reducing the number of lens elements, selecting the type of movement of lens units, such as a rear focusing system which is suitable for making the lens driving mechanism small and simple and effectively provides a stable imaging performance from the wide angle end to the telephoto end, and moreover reducing the thickness of each lens unit by reducing the thickness of each lens element. In addition, by selecting appropriate arrangement and structure of the filters, the depth of the optical system as a whole is thoroughly minimized.